extconf.rb: have_header

* ext/tk/extconf.rb (find_tcltk_header): use have_header instead of
  try_cpp, which is incredibly slow with VC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38760 b2dd03c8-39d4-4d8f-98ff-823fe69b080e